Abstract: A casing surrounding a fluid stream over which an unbounded fluid flows in a downstream direction has a plurality of alternating, adjoining troughs and ridges in its external surface, extending in the downstream direction to a thin trailing edge of the casing, which will thereby have a wave-like shape.

Abstract: A body adapted to move downstream through a fluid has a downstream extending smooth surface terminating at a blunt base. Extending transversely across the smooth surface and disposed upstream of the blunt base is a thin, downstream extending plate closely spaced from the smooth surface. At least the downstream end portion of the plate is convoluted such that its downstream end is wave shaped. The convolutions are designed to generate pairs of counterrotating vortices which delay boundary layer separation from the smooth surface and cause fluid to flow into the space immediately behind the blunt base, reducing base drag on the body. The device generates very little drag of its own.